Similar plastic parts are commonly produced in injection molds with single or multiple cavities. In the case of an injection molding machine wherein the mold has multiple cavities, it is known to use a hotrunner system to deliver the hot plastics material or melt from a melt plastification barrel of the machine to the cavities in the mold. The hotrunner system provides the plastic melt at a defined melt pressure and a controlled melt temperature to each mold cavity. In order to accomplish this objective, the hotrunner system commonly employs a heated manifold through which melt conduits extend and heated injection nozzles.
Nozzle valve gates are used in the aforementioned melt distribution systems to control the opening and closing of gate orifices, that is, the orifices that open into each mold cavity and through which the melt is delivered. The valve gate is a positive shut off device that has an open and closed position. At the beginning of melt injection, a valve pin of the valve gate opens the orifice in order to allow the plastic melt to fill the adjacent cavity. In addition, after the cavity has been filled, the gate orifice remains open during a packing phase which relies on packing pressure to control the quality of the plastic part. While the thermoplastic melt starts to solidify during the packing phase, the valve gate closes the orifice to achieve a clean gate mark on the plastic part surface and to avoid stringing or drooling of melt through the gate from the hotrunner system while the mold opens for part injection.
A melt channel or passage is formed in the nozzle of the valve gate to deliver the hot plastics melt to the gate orifice. Movement of the valve pin inside this melt channel is generally an open and closed stroke in the axial or longitudinal direction of the nozzle. The valve pin is actuated between open and closed positions by means of a valve actuator that is connected to a rear end of the valve pin. With known hotrunner systems, the valve actuator is commonly located externally of the heated components of the hotrunner system (for example, the manifold) because the commonly used valve actuators are not functional at the usual melt processing temperature of thermoplastics materials which is between 200 and 450° C. Generally pneumatic and hydraulic valve actuators are provided with seals between the pistons and their respective cylinders that operate only below 200° C. Also, electromechanic actuators require a low ambient temperature of less than 200° C. It will be understood that a heated melt distribution system or hotrunner system inside a valve gate mold can, depending on the location of the actuators, affect the valve actuators by heat conductivity, radiation and convection. Because of this effect, valve actuators are commonly positioned at a sufficient distance from the heated surface of the melt distribution manifold and the injection nozzle to keep them within their operating temperature range, which is preferably below 100° C. Known valve pin actuators can be physically separated from the heated manifold and the injection nozzle or nozzles by various means which allow the actuators to be located in a remote location where the actuator temperature can be maintained below 100° C. In addition to this thermal separation from the hotrunner manifold and the nozzles, it is known to provide for direct or indirect cooling of the actuators. Thus a cooling circuit within the injection mold can be directly or indirectly connected with the actuator to withdraw heat from the actuator.
It is also known to provide injection molds with a high number of cavities for making small plastic parts and it is advantageous to make such a mold as compact as possible. However, it is difficult and costly to integrate valve actuators with an effective cooling system in a compact mold of this type. Generally, valve pin actuators require considerable space inside an injection mold and they can add to the overall stack height of the mold. Moreover, forming cutout spaces for the actuators and bores or cutouts for cooling lines as well as air, hydraulic, or electric lines weakens the mold plate structure that has to support the substantial forces of the melt injection pressure inside the mold cavities and the clamping force in the molding machine.
U.S. Pat. No. 5,948,448 issued Sep. 7, 1999 to Eurotool, Inc., describes a hotrunner system for injecting hot plastics material into an injection mold that includes a thermally insulated manifold with a plastics flow channel extending therethrough to a nozzle part. An elongated valve pin extends through the manifold and through the nozzle and it is adapted to open or close an injection aperture. The head of the pin is connected to an actuator located above the manifold and on the side thereof opposite the nozzle member. The valve pin is slidably mounted in an aperture formed in a valve seal bushing which is screwable fixed within a complementary bore in a top surface of the manifold.
U.S. Pat. No. 6,159,000 issued Dec. 12, 2000 to Husky Injection Molding Systems Ltd. describes a hotrunner valve gated injection molding device which directs melt from a melt channel to a melt cavity. A guide sleeve is positioned at the gate end of the nozzle body and surrounds a valve stem in order to guide the valve stem inside the guide sleeve and to provide a sealing device at the gate end of the nozzle body. In this known system, the actuator for the valve pin is mounted in a valve plate through which the nozzle body extends and that is separate from the hotrunner manifold. The guide sleeve at the forward end of the nozzle assembly may be made of any high resistant tool steel and can be a nickel/chrome tool steel with a gas nitriding surface treatment to harden the surface, or a tool steel having hard wearing properties. There can be a close tolerance sliding engagement of the valve stem inside the guide sleeve which is said to inhibit leaking of plastic melt through the bore in the sleeve.
There is disclosed herein a novel valve gate apparatus for delivering and injecting hot plastics material into an injection mold. This valve gate is provided with an elastomeric wiper seal extending around and engaging the valve pin adjacent a machined guide bore. This wiper seal is made of a wear resistant material and is capable of withstanding operating temperatures for the valve gate apparatus of at least 200° C.